While the particular invention has specific application to motor graders, it could also be used on other vehicles having a need for four-wheel traction. The general concept of an auxiliary hydraulic front wheel drive working in conjunction with a conventional mechanical transmission which provides the primary drive is well known in the art as exemplified in U.S. Pat. Nos. 3,354,978 and 3,339,660. Most of these auxiliary systems previously taught in the prior art utilize a totally independent hydraulic system whereby the auxiliary system has its own pump source. The auxiliary system of the present invention makes use of a pump source from another system, more specifically, it utilizes the pump source used to power the conventional hydraulically driven accessories on the vehicle. In the specific application of a motor grader, these accessories would include the various power assists for braking, steering and the various cylinders for positioning the scraper blade. Since the pump source in modern variable displacement systems comprises the heart of the system, the elimination of a pump source on a system has its obvious economic advantages.